Thursday's London conference on Somalia is portrayed as a bold attempt, in the words of the foreign secretary, William Hague, "to change the dynamic from one of inexorable decline to an upwards trajectory of gradually increasing stability". The hope is to diminish the considerable internal and external dangers posed by "the world's most failed state". This is a wholly commendable aim.

But by raising expectations and setting a timetable and targets for political reform, security assistance and regional collaboration that are unlikely to be met, Britain and its partners risk making a bad situation worse. Without determined follow-through, these good intentions could open the way to greater human suffering, increased foreign military intervention and, ultimately, partition – presaging the definitive disintegration of Somalia as a sovereign state.

The conference communique, drafts of which have been widely leaked, recognises, in effect, that the ramshackle, temporary governance arrangements in place since 2004 have not worked and are no longer sustainable. "Nobody would agree to the roll-over of the transitional federal institutions in August" when their mandate expires, the draft communique says.

The fact the eight-year-old pretence that Somalia has a functioning, legitimate government is finally being dropped is a relief. During its existence, the so-called transitional federal government (TFG) rarely extended its rule much beyond a portion of the capital, Mogadishu, despite financial and administrative assistance from Britain, the US and others and military support from Ethiopia and the African Union (AU). During this period, Somalia became the most corrupt and least accountable country in the world, according to Transparency International.

Yet the conference plan to hand control in August to another, as yet undefined, "caretaker authority" risks a potentially catastrophic power vacuum. This authority would notionally be in charge until a new constitution has been written and endorsed in a referendum, nationwide elections held, and a new president, prime minister and parliament installed. This scenario may make sense to Whitehall mandarins. But in the semi-anarchic, on-the-ground Somali context, it is fantasy politics.

Somali leaders including President Sheikh Sharif Sheikh Ahmed, pro-government militia, and regional representatives met last week in Garowe, capital of Puntland (a self-declared autonomous state since 1998 that remains notionally part of Somalia), and agreed new federal arrangements and a reformed parliament. The plan was endorsed by the UN and AU. But key players, notably the Islamists of al-Shabaab, successors to the more moderate Islamic courts supplanted by the TFG, were absent. Exactly how these new governance arrangements are to be effectively implemented when, like the Taliban in Afghanistan, the main opposition force is excluded from their formulation defies rational explanation.

Matt Baugh, Britain's ambassador to Somalia, quoted in an Ethiopian government newsletter, welcomed the agreement but warned that all parties would have to "deliver on what they have said they are going to do". But delivering on promises is a perennial problem in Somali politics (as elsewhere). The London meeting is adding to the pile of pledges without adding to confidence they can be fulfilled.

One supposed answer to the question of implementation is contained in another conference proposal: that the UN-backed peacekeeping force, Amisom, be expanded to almost 18,000 troops. But once again, ambition may outstrip reality. The AU has struggled for years to attract troop contributors in Somalia. Where these additional forces may suddenly come from is up in the air. Maybe Djibouti; maybe Sierra Leone. The EU may pay for them; then again, it may not. The history of UN peacekeeping worldwide is one of bills unpaid and resources over-stretched. And given its chaotic recent history, Somalia is a particularly unattractive proposition for blue helmets.

In many other respects, the London conference is more a wish-list than an action plan. Somalia's prime minister, Abdiweli Muhammed Ali, said last week he had high hopes. "We expect a complete reconstruction plan for Somalia. We expect a Marshall Plan." His expectations will certainly be dashed. Likewise, feelgood proposals for wider grassroots political consultation and engagement, for co-ordinated international financial and economic support for the Somali regions, for "more generous" humanitarian responses, and for enhanced regional collaboration on terrorism and intelligence-gathering are entirely laudable – but unattainable in the countdown to the August handover, and quite possibly impossible amid the uncertainty that will then ensue.

Somalia cannot afford another political failure. But if the London process does fail, because it is too ambitious or because, as in the past, sufficient political and practical follow-through is lacking, the country will once again be given over to rule by brute force. This prospect embraces not only al-Shabaab militants and other lawless Somali militias, clans and separatists, but also US and British forces primarily concerned to suppress piracy and al-Qaida-linked terrorism, and regional powers such as Ethiopia and Kenya anxious to secure their borders. Somalia could become an international hunting ground, prey to all who deem their interests served by physical intervention.

The longer the Kenyans stay, the stronger this prospect looms. And with this comes the growing likelihood that the self-governing Galmudug region, in central Somalia, may follow suit or that the Ethiopians, worried about ethnic Somali insurgents in the Ogaden, may intervene again. In the continuing absence of effective central government, it is but a short step from this sort of free for all to permanent partition and the de facto end of the state of Somalia.

Hague is right to suggest this could be Somalia's last chance. The state as presently constituted has only existed since 1960. The preceding decades, going back to the 1880s, were marked by persistent British (and Italian) colonial incursions. Ironic then, and possibly fitting, that more modern-day British meddling may precipitate Somalia's ultimate demise.